The A/C stopped working over the winter. The compressor does not seem to be running. I removed the control unit compressor cutoff back behind the battery (part number 002 545 12 05).
1. Connected terminals 5 and 7 on the 12-pin connector; started engine, and detected compressor was running, I think. I did detect some change when I removed the jumper. With the engine off but the key on, connecting the jumper caused a click of the compressor's electromagnetic clutch.
2. The RPM sensor checked out OK (0.53vAC between terminals 9 and 11 of the 12-pin connector with the jumper still in place between terminals 5 and 7).
3. Transistorized rpm signal checked out OK (8.4 VDC between terminals 1 and 2 of the 12-pin connector with jumper still inplace between terminals 5 and 7).
I also checked the "refrigerant compressor pressure switch" which is down by the bottle with the little glass window in it. I believe this switch is supposed to make if there is pressure in the lines once the compressor start running. But since the compressor did not seem to be running, this did not seem to be valid test. Shorting out this little switch's terminals did not cause the compressor to run. (For this test I replaced the control unit in its 12-pin connector, connected a jumper across the pressure switch's terminals, started the car, turned on the A/C with temp set to minimum, still the compressor did not run.)
4. The temperature switch was not grounding out pin 12 of the 12-pin connector, I checked.
5. With the A/C turned on and temperature set to minimum, the input side of the "refrigerant compressor pressure switch" was grounded.

My questions follow:
The control unit (part number 002 545 12 05) normally energizes the electromagnetic clutch on the compressor to get it started, but what input to the control unit tells it to energize the electromagnetic clutch? The only input I can see from the push button switch unit in the dashboard goes to the "refrigerant compressor pressure switch." But for this signal to get through the pressure switch, the compressor has to be running. So what gets the compressor running in the first place?

Is there a sensor that detects the level of coolant in the system? (My A/C was converted over to non-R12 stuff last March, and it worked great all summer in Tucson. I don't think this has anything to do with the A/C not working, but thought I'd mention it.)



Now it is the next morning. I don't have a way to test the pressure - will have to take it to the shop (Mark at Benztek in Tucson) probably to get that checked out. Still this seems electrical to me. Shouldn't the compressor run for a little while even if the coolant is all leaked out?

Now it is a few days later. I have confirmed that the system was converted to R134 a little over a year ago, and the A/C worked fine over the long Tucson summer.

I am beginning to think that the pressure switch down by the dryer? (bottle with the little window in it) should be always "made" if there is freon stuff in the system. This would explain how the "signal" from the a/c controls inside the car gets through the switch and on to the control unit behind the battery before the compressor starts running. So if the pressure switch is not "made" (mine is always open) that means there is not enough coolant or freon like stuff in the system. If that was the only problem though, I would have expected the following test to have caused the compressor to run: (For this test I replaced the control unit in its 12-pin connector, connected a jumper across the pressure switch's terminals, started the car, turned on the A/C with temp set to minimum, still the compressor did not run.) So I need to check the R134 level. If that is good and the pressure switch is good, then the control unit must be bad.

I have read in some posts a couple of years old which say R134 is a real problem. Is that still the case?

I took the car back to Mark at Benztek in Tucson. All the R134 had leaked out and some oil was leaking out too. Mark found some seals (I think, sorry, I'm no expert) near the compressor, like above it maybe, that were leaking. He replaced those and refilled the R134. He didn't charge me since he is not sure that there is not a leak at the compressor (which was not changed during the R12 -> R134 conversion). I'll drive it for a few days and have a look under the compressor, it should be obvious if oil is leaking as it's all cleaned up now.

9.8 times out of 10, if the compressor stops working for no apparent reason it did so because of low pressure caused by loss of freon.

You've got an honest mechanic there - usually they take this opportunity to drain your bank account )which he still may do if you still have a leak.

did they flush out all the R12 mineral oil before conversion? R134a pag oil does not mix well with mineral oil. alot of shops just screw on the r134a fittings and charge up and call this a converssion!

__________________
1993 Mercedes Benz 300D 2.5 205K (ex wife's)
1984 Mercedes Benz 300SD 320K (SOLD)
2004 Mercedes Benz C240 75K
1995 GMC Sierra 2WD 5.7L 188K
1983 Mercedes Benz 300SD 239K (SOLD)
1987 BMW 325i 220K (SOLD FOR SALVAGE)

609 Certification

Didn't read it all, but your interpretation of the control circuit is wrong. The control does come from the push button controller and pass through the pressure switch to the compressor relay.

What you got wrong is the action of the switch. The switch is closed by normal refrigerant pressure. Normal being just about any amount in the system up to way too much. A non running system will contain refrigerant at around 100psi (depending on temp).

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

I have an 88 2.3 190e, its obvious a conversion has been done,which leads to the my issue.
Needless to say,I have'nt this car long and its getting hot out so the a/c need to operative.So while while trying to recharge my a/c with an can of "134 mix" the compressor came on about 3sec into the refill then shut off.I tried jumping and still no action.
Should I move on to finding out which sensors are bad or is this a compressor issue?

BCM,
My 190E manual says "On vehicles as from model year 85 the refrigerant compressor will be activated only after approx. 10 seconds as soon as an engine speed of approx. 600 rpm has been attained (to stabalize the engine speed).

"The control unit compares the two speeds of the ring gear and the refrigerant compressor 2 seconds after the compressor comes on (to compensate for clutch slippage during cut-in). If there is an rmp difference of more than 30% as soon as the compressor comes on, this condition is checked for a period of 200 milliseconds. If the rmp difference equalizes within 200 milliseconds, the compressor remains engaged. If the rpm difference remains, the control unit will immediately cut out the compressor. This process is again repeated when the ignition is switched off and the engine is restarted."

The rpm difference could be due to slippage of the belt or a bad rpm sensor in or at the compressor (not sure exactly where that sensor is). The control unit (behind the battery) could also be bad.

You can remove the control unit and force the compressor to run by putting a wire jumper between pins 5 and 7 of the 12-pin connector where the control unit plugs-in. (Be careful as you can see some sparks if you short pin 5 to ground.) Then measure that the rpm sensor is working with a voltmeter between pins 9 and 11 of the 12-pin connnector and set the voltmeter to AC voltage. It should read about 0.3 vAC at approx 750 rpm. Mine read 0.53 vAC and was OK. Move the voltmeter to pins 1 and 2 of the 12-pin connector, set the voltmeter to DC volts, and you should see approx 8.5V. Mine read 8.4 volts.

If the belt is tight and not slipping, and these voltmeter readings are OK, it could be the control unit, I guess, but don't quote me. Like I said before, I am no expert.

Mark at Benztek is good and perhaps excellent and is honest, and he is listed on this site. I will no doubt be back in his shop for future repairs. He tries to keep the cost down by doing the right thing; his repairs are sound.

This finally dawned on me.

The oil was replaced with an oil that is compatible with either R12 or R134.